Employees
| Concept | As at 2025-03-31 | As at 2025-03-31 | As at 2025-03-31 | As at 2025-03-31 | As at 2025-03-31 | As at 2025-03-31 | As at 2024-03-31 | As at 2024-03-31 | As at 2023-03-31 | As at 2023-03-31 | As at 2022-03-31 | As at 2022-03-31 | As at 2021-03-31 | As at 2021-03-31 | 
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Employees | ||||||||||||||
| Information about reporting company | ||||||||||||||
| Information about employees of reporting company | ||||||||||||||
| Number of employees | 
                                                1
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                1,125
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                1
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                1,102
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                1,126
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                1,103
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                1,124
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                1,135
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                1,117
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                1,130
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                1,152
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                1,164
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                1,215
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                1,207
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
| Average number of temporary employees | 
                                                
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                171
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                168
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                171
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                168
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                172
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                173
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                192
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                194
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                211
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                213
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                225
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
                                                224
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | 
| Average age (Years) | — | — | — | — | — | 
                                                41.8
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | — | — | — | — | — | — | — | — | 
| Average length of service (Years) | — | — | — | — | — | 
                                                17.6
                                                
                                                    
                                                        xbrli:pure
                                                    
                                                
                                             | — | — | — | — | — | — | — | — | 
| Average annual salary | — | — | — | — | — | 
                                                8,363,172
                                                
                                                    
                                                        JPY
                                                    
                                                
                                             | — | — | — | — | — | — | — | — |